Subsea cables are commonly used to transfer power and/or data between remote locations. The cables are typically laid along the seabed to avoid the need for complex supports or cable routing. They may be buried in the seabed and/or covered, for example, with concrete plates. Over time, cables may fail or become damaged. As those skilled in the art, damage to subsea cables will typically be in a specific location on the cable.
The operation of repairing damaged or faulty cable under the sea is challenging. As a result, the repair of a faulty underwater cable typically takes place on a specialist repair vessel, and broadly involves the following steps:                1. Pinpointing exactly where the fault is        2. Removal of any burial or protection (e.g. concrete panels) placed over the cable for distances of sufficient length to permit repair on either side of the fault        3. Removal of a section of cable by cutting either side of the fault location using a suitably equipped remotely operated vehicle (ROV);        4. Recovery, to a vessel, of each end of the cable in turn;        5. Addition of a new section of cable between the cut ends; and        6. Return of the repaired section to the seabed.        
The length of the new section of cable added as part of the repair is dictated by the water depth at the location of the fault. Generally speaking, the additional length required is approximately equal to three times the water depth at the repair location.
To ensure that the faulty section of cable has been removed, the repair method can also include the step of inspecting and testing a first recovered cut end of the cable being on board the vessel before returning it to the sea, marked with a buoy, while the second cut end is recovered. This requires the additional steps of applying a temporary protective coating to the first cut end of the cable to protect it once it is returned to the water, and subsequently removing this protective coating when completing the repair. In addition, if the first end is returned to the seabed it will need to be recovered twice. Each recovery operation can take as long as twenty-four hours to complete, slowing the repair operation and increasing the overall cost. The recovery operation is typically performed using a grapnel and this can put strain on the cable. Further, the recovery operation can also cause damage to the cable directly, so there is a risk that the cable end could acquire further damage on being recovered from the seabed.
It will be appreciated that repair of the cable will typically involve the insertion of a section of new cable being used in a repair operation. This can be problematic where the initial cuts are incorrectly located, so that additional cable has to be cut from one end to remove the fault following testing or inspection. Improvements in the precision of fault location have reduced the amount of existing cable that will typically be cut and discarded when a fault has been determined. However, with the existing repair systems, a significant amount of additional cable is still required to conduct the repair, particularly in deep water areas.
This additional cable has to be stored and carried on the vessel making the repairs, and additionally results in large loops of surplus cable when the repaired cable is returned to the seabed following the repair, neither of which is ideal. The need for long lengths of additional cable also increases repair costs, and problems with cable supply can lead to repairs being delayed. Repair loops, being perpendicular to the line of the cable, may also fall outside the consented corridor and bring complications and timing issues whilst requisite consent is achieved from the relevant parties.